Here's some insight for the CB750:
The rear of the engine is the 'box' for the frame. There are 3 major bolts there: the upper engine bolt, the lower engine/footpegs rod, and the swingarm bolt. These 3 must be tightened first when assembling the 'system'. It causes the middle uprights of the frame to twist inward, rigidizing the whole middle of the bike. Then the lower and front bolts of the engine get tightened, and the engine stays perfectly square. If the front bolts are tightened first, the rear wheel can end up off-center ever so slightly, as the swingarm's pivot is not square to the frame. This will be felt in the bike as a push-back in the right-hand grip when you are riding.
The engine cases twist a lot during hard runs. So much so, that if you have a post-1972 750 with the dual-row output bearing on the final drive, and it is not well-worn in, it loses a couple of HP to binding in the lower gears at a hard run. This is why the single-row final drive was/is coveted by the drag racers and roadracers. In most of these engines, they don't run as easily under hard push until they have enough miles on them to not bind the transmission bearings under all this twist: the so-called "high-speed" ball bearings on one side of the tranny shafts were/are used to reduce this bind under power: they are nothing more than bearings with one less ball than normal, and with .0003"-.0005" more inner race clearance. Once these bearings wear in well, they last for a very long time because they don't support much load, they just hold the alignments "like a willow tree".
You can 'see' these flexes even by hand when you have the cases apart: set a dial indicator to the left front corner of the lower case with it clamped ell to a mill, then press the right rear corner with your thumb, hard. You will see the distortion show up, every time! This makes it real tricky to align-bore the main crank bearings, and it explains perfectly why the bearings virtually stop wearing after they reach "normalization", somewhere around .0016" clearance, typically. When they have worn enough to let the cases flex around the crank (which DOESN'T flex much), there is enough support for the whole system.
The 750 also has a small cylinder base-to-height ratio that would not seal well if the cylinders were stressed as in the CB/CL Hawk series bikes, or the CBX. There would have to be additional seals added at the base to contain the oil, although this could be done. If you've ever built a SuperHawk engine, you know just how hard it is to separate those cylinders from the case, and how easy it is to have them leak if you don't torque it all back together just right. That section of those engines is heavily overbuilt, to create the missing downtube of those frames.
